ELSEVIER The Science of the Total Environment 180 (1996) 187-200 Modelling the long-term changes in stream, soil and ground water chemistry for an acid moorland in the Welsh uplands: the influence of variations in chemical weathering M. Cristina Forti”>*, Colin Nealb, Alice J. Robsonb ‘Insiituto National da Pesquisas Espaciais - INPE, CP 515, Sao Jose dos Campos, CEP 12201-970. Sao Paula, Brazil bInstitute of Hydrology, Maclean Building, Crowmarsh Gifford, Wallingford, Oxon, OX10 888, UK Received 30 June 1995; accepted 16 August 1995 Abstract A long-term model of stream acidification, MAGIC, has been applied to an acidic moorland catchment, the Gwy, in mid-Wales. This application has been used to examine the effects of variability of weathering on soil, groundwater and stream water chemistry from 1844 to the present day and onwards to 2080 with a predicted acidic oxide reduction of 60% in the future. The results show that weathering initially affects soil, groundwater and stream water chemistry profoundly. Despite this, the results indicate that a simple two-layer (one soil and one groundwater end-member) model can still provide a good prediction of long-term stream water quality even when each soil and groundwater area is heterogeneous in composition. The work provides support for the use of the lumped MAGIC model. However,for the predictions to be applicable it is shown that it is critical that the relative contribution of waters and soil exchange materials from the hydrochemically distinct regions within the catchment are adequately represented within the model. This probably means that further field work is required to examine source area contributions. Keywords: Heterogeneity; Weathering; Modelling; Soil water; Groundwater 1. Introduction Rain is involved in a complex but poorly defined mixture of physical, chemical and biological pro- cesses within the soil and groundwater areas on transport through catchments to generate a stream flow response. These processes control the surface drainage water chemistry and this critically im- * Corresponding author. pacts on the vitality of stream ecology (Neal et al., 1994).Major studiesover two decades have shown that acidic deposition and land use change over large tracts of acidic and acid sensitive soils have affected stream water quality to the detriment of stream ecology (Harriman et al., 1994). Conse- quently, major efforts are now being made on ex- amining potential remedial solutions and in assessing the potential of emission and land use control strategies. 0048-9697/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0048-9697(95)04935-G
Transcript
ELSEVIER The Science of the Total Environment 180 (1996) 187-200
Modelling the long-term changes in stream, soil and ground water chemistry for an acid moorland in the Welsh uplands: the influence of variations in chemical weathering
M. Cristina Forti”>*, Colin Nealb, Alice J. Robsonb ‘Insiituto National da Pesquisas Espaciais - INPE, CP 515, Sao Jose dos Campos, CEP 12201-970. Sao Paula, Brazil
bInstitute of Hydrology, Maclean Building, Crowmarsh Gifford, Wallingford, Oxon, OX10 888, UK
Received 30 June 1995; accepted 16 August 1995
Abstract
A long-term model of stream acidification, MAGIC, has been applied to an acidic moorland catchment, the Gwy, in mid-Wales. This application has been used to examine the effects of variability of weathering on soil, groundwater and stream water chemistry from 1844 to the present day and onwards to 2080 with a predicted acidic oxide reduction of 60% in the future. The results show that weathering initially affects soil, groundwater and stream water chemistry profoundly. Despite this, the results indicate that a simple two-layer (one soil and one groundwater end-member) model can still provide a good prediction of long-term stream water quality even when each soil and groundwater area is heterogeneous in composition. The work provides support for the use of the lumped MAGIC model. However, for the predictions to be applicable it is shown that it is critical that the relative contribution of waters and soil exchange materials from the hydrochemically distinct regions within the catchment are adequately represented within the model. This probably means that further field work is required to examine source area contributions.
Rain is involved in a complex but poorly defined mixture of physical, chemical and biological pro- cesses within the soil and groundwater areas on transport through catchments to generate a stream flow response. These processes control the surface drainage water chemistry and this critically im-
* Corresponding author.
pacts on the vitality of stream ecology (Neal et al., 1994). Major studies over two decades have shown that acidic deposition and land use change over large tracts of acidic and acid sensitive soils have affected stream water quality to the detriment of stream ecology (Harriman et al., 1994). Conse- quently, major efforts are now being made on ex- amining potential remedial solutions and in assessing the potential of emission and land use control strategies.
0048-9697/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0048-9697(95)04935-G
E8E M.C. Forti et al. /The Science of the Total Environmenr 180 (1996) 187-200
Central to planning new environmental manage- ment strategies is the development of reliable long- term bydrochemical models which describe the chemical transfers occurring in catchments. These models are needed to predict the effects of changes in atmospheric pollution and land use on the various chemical stores within catchments and hence on stream water chemistry. While the nature of catchments is such that they are complex and highly heterogeneous, in relation to soil and groundwater chemistry, the models are relatively simple in structure in that they ‘lump together such variability (Christophersen et al., 1993).
trix, as well as the soil water compositions; (2) chemical weathering is the major process by which acidity, generated from atmospheric pollution and from within the soils, is neutralized. Thus, weathering is the critical step in determining stream ecology vitality.
ile such lumping is necessary, given the complex environment being described and the ease sf overparameterising model structures, both theoretical and practical difficulties arise (Christophersen et al., 1993; Neal, 1995). The the- oretical difficulties occur because, in lumping, the underlying equations that apply at any instance at the local (e.g. micropore) scale are not necessarily applicable at the time averaged or the large (e.g. catchment) scale. Practical problems occur due to the difficulties of adequately measuring chemical variability and our inability in knowing how to in- tegrate the variability into catchment average values, on the basis of flux transfers, for use in the lumped models. There may be missing processes that account for acidity regulation in the soil and groundwaters, and field prograrnmes are still required to identify them (Neal, 1995).
In this paper, the question of the effects of weathering heterogeneity on stream water quality is addressed. This is done by the application of one of the most widely used long-term acidification models, MAGIC (Cosby et al., 1985a,b), to predict the evolution of stream water quality, under dif- ferent weathering regimes. The approach taken parallels that of the earlier desk studies which de- scribe the soil exchange variations. However, this new study advances the earlier studies by directly (1) relating the cation exchange variations to the weathering process, (2) producing an application for long-term simulation (the earlier studies took no account of long-term changes in the chemical stores within the soils) and (3) modelling a term for groundwater variability. The application is ap- plied to the Gwy, the main headwater catchment of the river Wye, a typical example of a British upland acid-moorland system. The results presented here update earlier work described in Robson et al. (1991) and Neal et al. (1992), in the light of further data.
2. Study area
Recent desk studies, following field measure- The Afon Gwy is the principle headwater tribu- ment of chemical variability, have highlighted the tary of the River Wye in the Plynlimon area of problems of spatial heterogeneity within catch- mid-Wales. The area, 24 miles inland of the west ment in terms of chemical variations within the coast of mid-Wales, forms part of a deeply soil (Neal, 1992; Neal et al., 1994). These studies dissected plateau, much of it over 450 m, rising to describe the impact of variations in the ex- 750 m on the summit, Pumlumon Fawr. The River changeable cation fraction of the soil matrix which Wye drains to the south-east of this area. Stream is a critical control in determining the regulation of channels are irregular in profile, and the steps soil water acidity by base cation supplies from the often coincide with bands of resistant rock. The soil surface and hence, on transfer to the stream, river valleys tend to be deeply incised and are stream water acidity. However, no adequate com- separated by broad boggy interfluves. Thus, the panion studies have been undertaken to examine landscape of the Plynlimon is dominated by rolling the effects of heterogeneous weathering reactions hills dissected by steep valleys. The Plynlimon in the soil and groundwater zones. This shortfall is massif, on which Plynlimon lies, is composed of potentially an important omission as (1) variations lower Palaeozoic rocks (Ordovician slates and in chemical weathering probably provide the main Silurian mudstones; Breward, 1990). Soil cover driving force for ensuring the high variations in consists of a mosaic of acid upland types including the exchangeable cation fractions of the soil ma- peats, brown earths, stagnogleys and stagnopod-
M. C. Forti et al. /The Science of the Total Environment 180 (1996) 187-200 !89
201s. The summits of the Plynlimon Massif have extensive areas of peat (depth around OS-l.5 m), with grassy areas over the rock pavements from which peat has been eroded. These peats probably exert an important effect in the control and storage of water (Knapp, 1970; Institute of Hydrology, 1972). Brown earths are present on the better- drained, steeper slopes. Podzols and stagnopod- 201s have developed on the mid-slopes, and gleys are present in the wetter, poorly drained areas of boulder clay. In the valley bottoms, peat bogs are common, with depths ranging between 0.8 and 1.2 m.
Drift deposits within the catchments are locally derived and consist of a stony boulder clay found mainly at the base of slopes, with shale or grit col- luvium on the slopes themselves.
Plynlimon has a cool and wet maritime climate, a result of its proximity to the Irish Sea and Atlan- tic Ocean. The Institute of Hydrology monitors climate and rainfall inputs using a network of automatic weather stations and ground level and canopy-level rain gauges (Institute of Hydrology, 1976). Mean monthly temperatures are lowest for February (l.S’C) and peak in August (13.1”C). Rainfall averages around 2500 mm/year but varies with altitude; the increase in average annual precipitation, per 100 m of ascent, is estimated to be 171 mm (Newson, 1976). Thus, the tops of the catchments may receive around 25% more rain than near the outflows. Sixty percent of rain falls between October and March, with rain occurring, on average, on 2 out of every 3 days over the winter. Seventy-five percent of rainfall is of the westerly synoptic type (Lamb, 1972) and 10% from cyclonic circulation. Five percent of precipitation falls as snow. Estimated annual evaporation for the forested areas is between 29% and 32%, of which around 25% occurs through interception loss and 4-7% through transpiration. Over grassland areas, evaporational losses are estimated to be around 15-17% (Hudson, 1988).
The Gwy catchment is 390 ha in area, with an altitude range of 380-730 m. Average rainfall is 2424 mm/year and average runoff is 2083 mm/year.
The dominant vegetation on the Gwy catchment is an acid Nardus-Festuca grassland and it oc- cupies some 56% of the catchment. This vegetation characterises the long well-drained slopes of acidic
podzol soils. Hill top peats, which CQW~ 37% of the catchment, support Eriophorum, C&ww and Vaccinium species and are highly acidic. The re- maining podzol of the catchment (7%) comprises improved grassland.
The episodic stream response of the Gwy catch- ment to rainfall is very flashy, influenced by the thin soils and steep slopes. The hydrological re- sponse is promoted by a range of mechanisms. The soils have high near-surface hydraulic conduc- tivities which transmit throughflow water quickly, and macropore flow and overland flow have been observed. Rainfall chemistry is variable but
dominated by sea salts given the predominance of westerly winds supplying rain from the Irish Sea and North Atlantic Stream chemistry is also in- fluenced by marine salts, but is highly damped re- lative to rainfall inputs (Robson, 1993). The stream chemistry is generally dilute but is acidic (pH - 4.8) poorly buffered and aluminium rich at high flows (Reynolds et al., 1986). At baseflow, the streams are well buffered (alkalinity - 65 pe- quiv./l) and of fairly high pH (pH - 6.5). This re- sponse may be broadly explained in terms of the varying contributions from waters which have been neutralized by bedrock, and from the acid soil waters.
3. The modelling approach
3.1. The MAGIC model MAGIC (Model of Acidification of Ground
waters In Catchments) is a model designed to ex- amine the long-term changes in stream and soil waters occurring in response to acidic inputs to the system (Cosby et al., 1985 a,b). MAGIC assumes that atmospheric deposition, mineral weathering and cation exchange processes in the soil and groundwater zones are responsible for the observed stream water chemistry in a catchment. A summary of the main processes represented in the model are as follows:
(1) Strong acid anion concentrations are calculated for the soils and stream waters. Sul- phate is assumed to follow a Langmuir isotherm (Singh, 1984)
Es = E, (S042-)/[C + (S0,2-)1
where E, is the maximum adsorption capacity of
190 h4.C. Forti et al. /The Science of the Total Environment 180 (1996) 187-200
the soils and C is half the saturation concentra- tion. This allows for the lags which can occur be- tween atmospheric deposition and the resultant changes in stream water sulphate concentrations. Chloride, nitrate and fluoride are assumed not to have an adsorbed phase. Loss of ammonia and ni- trate to the vegetation is included within the model by estimating a percentage uptake.
(2) Cation exchange processes involving alumin- ium, sodium, calcium, magnesium and potassium are assumed to be operative. The general form for the exchange reactions between the ions IV”‘+ and N”+ and their adsorbed states MX,,, and NX,, is
n Mm+ + m NX, = m N”+ + n MX,,,
The total cation exchange capacity is then defin- ed as
CEC=C
Exchangeable cations. Equilibrium expressions for the cation reactions
are approximated by
SMUIN = ( N”+ ) m En&( Mm+ ) n EmN
where Sr,,rVIN is the selectivity coefficient between M and N; ( ) refers to the chemical activity of an ion in solution, and E is the exchangeable fraction for the appropriate ion on the soil complex, e.g.
EN = XKEC
with Xequalling the amount of adsorbed cation N. (3) Aluminium concentrations are assumed to
be in equilibrium with the solid phase Al(OH)s.
3 H+ + Al(OH)s(s) = A13+ + 3 HZ0
(4) Allowance is made for dissolution of car- bonic acids caused by the elevated Pco2 levels in the soils, and for the effects of degassing as water moves from the soils to the stream.
(5) It is assumed that there are constant long- term net inputs of base cations from mineral weathering. These are difticult to measure and must therefore be estimated.
(6) Organics are modelled within MAGIC using a triprotic representation.
3.2. A two-layer MAGIC application A two-layer version of MAGIC (Jenkins and
Cosby, 1989) is used in the application, enabling the simulation of two chemically distinct waters. The top layer (here termed ‘soil layer’) represents the lumped 0, E, B and C podzol horizons and the peats of the area, whilst the bottom layer (termed ‘groundwater layer’) represents the deeper till layers and other water stores such as groundwater which produce baseflow. The flow proportions in- cluded in the model are those suggested by a two- component mixing approach outlined previously (Neal et al., 1990): all the rain passes through the soil layer and a portion of this percolate con- tributes directly into the stream, whilst the re- mainder is routed via the groundwater layer to the stream.
Rainfall chemical inputs to the catchment were taken as the rainfall flux adjusted for occult and dry deposition of sea-salts and anthropogenic sulphur compounds. Chloride is assumed to be conserved within the catchment. Sulphate is cur- rently assumed to be in near-equilibrium, so that the total sulphate input is equal to the stream out- put. Differences between the rainfall concentra- tions and the stream outflow concentrations (allowing for evaporation effects) are assumed to result from dry and occult deposition, and these components are incorporated within the adjusted rainfall. For the sulphate input, this ‘adjusted’ value (56 pequiv./l) lies near modelled estimates of total sulphate for the area (56 and 49 pequiv./l for 1987 and 1988, respectively; Robson, 1993). The rainfall inputs for the period 1844-1984 were varied according to the deposition sequence outlined by the Warren Spring Laboratory (1983).
The two layers in the model are conceptualised such that different reaction mechanisms dominate in each layer. The water from the soils is known to be organic and aluminium rich and acidic, in- dicating that ionic exchange mechanisms and or- ganic acid deprotonation reactions are the most important influence on soil water chemistry. Water from the groundwater sources is rich in base cations, but low in exchangeable cations, as it
M.C. Forti et al. /The Science of the Total Environment 180 (1996) 187-200 191
comes from a high weathering zone. In this appli- cation, it is therefore assumed that ion-exchange occurs in only the soil layer and that the ion ex- change capacity of the groundwater zone is small enough to be neglected. On the other hand, weathering takes place predominantly in the groundwater zone.
The soil characteristics used in the model are given in Table 1. The depth of the soil (0.9 m) cor- responds to the average combined depth of the 0, E, B and C horizons and of the peats. The bulk density and cation exchange of the soil is calculated from field data for the Gwy soils (Rob- son, 1993). An average depth of 1 m is assumed for the groundwater zone. The groundwater layer is represented as being denser and of lower porosity than the soil layer. The partial pressure of carbon dioxide is assumed to be 30 times atmospheric pressure for both the soil and the groundwater zones (Neal and Whitehead, 1988). For the stream, the PCO, is set to 2.5 times atmospheric pressure, in line with average observed values for the Plynlimon streams (Neal and Hill, 1994).
The concentration of organic acids in the two MAGIC layers is calculated from the average observed dissolved organic carbon, using a
Table 1 Soil characteristics used in the two-layer MAGIC model
Soil layer Groundwater Top layer Bottom zone layer
Depth Cm) Porosity Bulk density (kg/m3) E, (mequiv./kg) C (mequiv./m3) Cation Exchange Capacity
(mequiv.!kg) Temperature (“C) Pco, (atm)
‘%lo(KAl(oH)3) Exchangeable Cation (%) 1984: Na K
Mg Ca BS
0.9 1 0.55 0.35
800 1460 3 3
70 70 45 0. I
7.6 7.6 0.009 0.009 7.5 9.5
0.10 0.00 1.90 4.8 6.7
calibrated conversion formula from the pro- gramme of Schecher and Driscoll (1988), ALCHEMI (Robson, 1993): this conversion is equivalent to assuming 18 carbon atoms per mole- cule of organics. The organic dissociation con- stants used in MAGIC are specified to be the same in both the soil and groundwater zones; they match the default diprotic system values for the ALCHEMI programme.
Very little is known about the sulphate characteristics of the Gwy soils. The selected con- stants which describe the isotherms were chosen to be typical of values used in earlier MAGIC ap- plications to the uplands (Jenkins et al., 1988; Whitehead et al., 1988; Jenkins and Cosby, 1989). The same sulphate absorption isothe;m was used in the two layers.
The model was calibrated to the present day chemistry of the stream water, the soil water and the groundwater by adjusting weathering rates, uptake rates and the initial soil base saturation (Table 2). Given the limitations of the assumptions required within the mixing approach, and the ana- lytical error in the chemical measurements, a perfect match between the modelled and the observed chemical species in the stream is not
Table 2 Calibrated parameters values for the two-layer MAGIC appli- cation to the Gwy
Weathering (mequiv. Ca m2/year) (negative Mg values = uptake) Na
K Percentage uptake (of NH.,
the input to the layer) NO, Selectivity coefficients Ca
m Na K
Initial base saturation
(1844) Na K Mg Ca
5 47 0 35
-11 32 0 -15
98 0 15 80 -1.4
0.14 -1.35 -5.09 16.15
0.10 0.05 7.0 9.0
192 M. C. Forti et al. /The Science of the Total Environment 180 (1996) 187-200
achievable. The alkalinity gives a good indication of the overall composition of a water sample and, whereas some deviations in individual deter- minants are acceptable, it is important that the modelled alkalinity is accurate. The selectivity coefficients and initial base saturation were calibrated by running the model for the period 1844- 1984 so as to match present day base satura- tion characteristics of the upper soils and peats (Robson, 1993). Weathering/uptake rates were then adjusted for the two layers in order to match stream and end-member chemistries as closely as possible. Nitrate and ammonia were modelled by calibrating catchment uptake to match the differ- ence between the inputs and the outputs. Weathering-inputs/biological-uptake-rates were used to match observed base cation concen- trations.
Optimisation was performed manually, by ad- justing parameters so as to match present day stream chemistry as closely as possible.
The model has also been used to estimate future changes in stream, soil and groundwater com- ponents. This was done by continuing the already calibrated run for 1844- 1984, using a linear future sulphate deposition reduction scenario of 60% of present day levels, starting in 1984 and being com- pleted by the year 2084, and held at a constant level thereafter.
The mean annual proportion of soil and groundwater contributions to stream water was calculated to fix the flow pathways required for the MAGIC application. As MAGIC bases its predictions on knowledge of all of the major chemical determinants, not just alkalinity, it is also necessary to have a full estimate of present day end-member chemistries which can be used as a guideline within the model calibration. The approximate end-member chemistry of the soil waters is expected to lie between the weighted average of the grassland soil chemistry (incor- porating the 0, E, B and C horizons) and the peat water chemistry (Table 3). It is necessary to lump together the soil and peat waters so as to restrict the complexity of the MAGIC application. Al- though there are chemical differences between these waters, they are both very distinct from baseflow. Table 3 shows the likely range that each
of the determinants will fall within. The mean flow-weighted proportion of soil water for the Gwy is calculated to be 0.48 from the hydrograph split.
3.3. Two- by two-layer application To evaluate the soil heterogeneity within the
catchment as a function of variations in within- catchment weathering rates, the MAGIC model was applied in a separate way from the two-layer application described above. For this case, here termed the two by two application, the MAGIC model was applied using all the parameters set for the two-layer application with the exception that
(1) the weathering rate was allowed to vary by fixed amounts to cover a range spanning what might be observe in the field. The weathering rates set correspond to times three (M3) times two (M2), divided by two (D2) and divided by three (D3), the value obtained for the two-layer application.
(2) the exchangeable cations and base saturation levels for the soil under pristine (1844) conditions were varied to allow a model fit for the condition where the cation exchange selectivity coefficient was fixed to the values determined by the primary two-layer application (Table 4). In other words, it was taken that the thermodynamic constants for cation exchange were fixed and that any variations in weathering rate would be reflected solely in terms of changes in the soil water chemistry and base cation levels.
From these results, a time series of water quality determinands was constructed for the soil and groundwater areas. This was undertaken to il- lustrate the effects of variations in weathering rates on their chemistry. The stream water chemis- try for the mixed waters was then determined using these end-member compositions. To represent the largest potential variations that might be estimated for the stream (relative to the two-layer applica- tion) the M3 and D3 compositions were mixed. In order to do so, the mixing volumes for each layer had to be estimated and this was done by calculating a mixing volume for the two soil and two groundwaters such that the alkalinity matched the soil water, groundwater and stream water values for the two-layer model. This was required as equal contributions of each water type would
M.C. Forri et al. /The Science of the Total Environment I80 (1996) 187-200 193
Average values shown with ranges in parentheses; units are in pequiv./l except for aluminium and DOC (PmolIl) and pH (dimen- sionless); from Robson (1993).
lead to major differences in chemistry compared to the two-layer application: a situation which is not allowable as the simulation must match the field observation for the stream.
For the calculation, pH was estimated directly
Table 4 Base saturation levels (BS%) and exchangeable cation percen- tages for the soil under pristine conditions (1844) for the two by two-layer model corresponding to weathering rates times
from the alkalinity because, on mixing of waters from the different areas, pH (and H+) does not behave conservatively whereas alkalinity, strong acid anions and strong base cations do. The pH for the mixed stream, soil water and groundwater was calculated assuming the same PCO~ levels as in the two-layer case.
4. Results
4.1. The two-layer case The modelled chemistry of the stream, soil water
and groundwater together with the exchangeable cation data for the soils compares well with the observed values (Table 5) and previously publish- ed data (Robson et al., 1991; Robson, 1993). Stream, soil and baseflow alkalinity are all well simulated by the calibrated model. Individual stream chemical determinands are well matched
194 M.C. Forti et al. /The Science of the Total Environment 180 (1996) 187-200
Table 5 Modelled stream, soil water and groundwater chemistry for the two-layer model
and lie within 1 pequiv.il of the observed stream chemistry, whilst for the soil layer, modelled soil water concentrations lie inside the observed ranges. For the groundwater, the modelled con- centrations are also satisfactory.
The time trends of reconstructed soil, ground and stream water chemistry from 140 years ago, that is pre-acidification, up to 2084, are shown in Fig. 1. The alkalinity shows a decline from 1844 to the 1980s and recovery thereafter. Initial recovery is fast and is followed by a very gradual long-term improvement in alkalinity.
The soil, groundwater and stream alkalinity ap- proximately parallel one another throughout the
PH
SULFATE
r CI,. I I I1 1. I 1844 1884 1924 1964 2004 2044 2084
MAGNESIUM
i I I 1 I I I . I . I 1844 1884 1924 1964 2004 2044 2084
Fig. 1. Time trends of reconstructed soil, ground and stream water chemistry (Alkalinity, Ca, SO, and Mg in mequiv.il, Base Saturation (%) and pH) from 1844 up to 2084.
M.C. Forti et al. /The Science of the Total Environment 180 (1996) 187-200 195
modelled record. Stream pH falls rapidly as de- position increases in the 1950s levels out in the 1980s and then rises in the future in response to increase followed by decrease in atmospheric de- position. The stream chemistry follows a similar pattern to the soil and groundwater, but the pH variations are greater owing to the lower partial pressure of carbon dioxide in the stream. As a con- sequence, CO2 degassing occurs as water moves from catchment to stream, and the pH of the stream is not bounded by the two end-member pH values, especially at higher pH levels. Sulphate concentrations in the stream, soil and groundwater change in line with the assumed changes in sul- phate deposition. The groundwater takes longer to adjust to changes in sulphate concentration be- cause it is replenished from the soil and is therefore only indirectly linked to the rainwater. The stream
(al
concentration represents a direct mix of the soil and groundwater. Predicted future sulphate con- centrations decrease in both the soil and ground- water, with a reversal in the sulphate concentration gradient between the soil and groundwater. Modelled groundwater sulphate concentrations are lower than in the upper soils for the period prior to 1970. In future years, sul- phate concentration in groundwater takes longer to decline than in the upper soil water.
4.2. The heterogeneous two- by two-layer case The effect on soil and groundwater chemistry of
varying weathering rates is very large. However, the shape of the time series patterns broadly follows that described for the two-layer case (Fig. 2a-c). In detail, there are some deviations in pat- tern. For example, in the case of alkalinity, the
Fig. 2. Time trends for the two-layer MAGIC model for different weathering rate values: times three (M3), times two (M2), divided by three (D3) and divided by two (D2). (a) Alkalinity (mequiv./l) and pH; (b) Magnesium and Calcium (mequiv./l) and (c) Sodium (mequiv./l) and Base Saturation (%).
196 44. C. Forti et al. /The Science of the Total Environment 180 (1996) 187-200
M. C. Forti et al. /The Science of the Total Environment 180 (1996) 187-200 197
time of minimum alkalinity varies according to the weathering rate, the position occurring earlier for the lower weathering rates. For both the soil and groundwaters, the higher the weathering rate, the higher the alkalinity and base cation concentra- tions, released by weathering, and the lower the pH. For the soil, where there is a significant cation exchange fraction on the soil matrix, the ex- changeable base cation levels increase as weather- ing increases (Fig. 2~).
With regards to the stream water composition, the effects of mixing the M3 and D3 soil waters and groundwaters under the condition that the mixed waters matches the 1984 alkalinity and pH, is such as to systematically lower the pH and the alkalinity, calcium and magnesium concentrations under the pristine and future times relative to the 1984 value (Fig. 3). The deviations increase both backwards and forwards in time around the 1984 point. Despite this, there is a very close cor-
Fig. 3. Time trends for stream water chemistry for the two-layer and two- by two-layer model from 1844 to 2084. Units are mequivA except for pH.
198 M.C. Forti et al. /The Science of the Total Environment 180 (1996) 187-200
respondence to the two-layer and the two- by two- layer models; the results are the same, for practical purposes.
5. Discussion
The results presented here show that variations in weathering rate will have a very strong influence on soil and groundwater chemistry. Changes in weathering rate change cation supplies to the soil solution. Thus increasing or decreasing the weathering rate provides corresponding increases or decreases in base cations and alkalinity.
In nature, there is a large variation in soil and groundwater compositions and, in part, variations in weathering clearly can have a substantial influ- ence. Despite the heterogeneity, the integrated value, characterized by the stream chemistry, in- dicates that, volumetrically, the extremes observed in the soil and groundwater areas may well have little influence on the modelled values as they counterbalance each other out. For this reason, it may well be argued that there is no need for undue effort in trying to match models to include these extremes. Furthermore, if one tries to recalibrate the two-layer model using weathering rates higher or lower than the optimised value, it becomes more and more difficult to fit the averaged base ca- tion levels. Thus it seems that the model has pro- vided a means of estimating the gross weathering rate reasonably well.
Table 6 Calibrated parameters values for the MAGIC application of the recalibrated two-layer model
Percentage uptake (of NH, 98 0 the input to the layer) NO, 15 80
K 6 Selectivity coefficients Ca -1.9
Mg -0.19 Na 3.01 K -1.76
Initial base saturation
W44) 42.21 Na 0.11 K 0.10 Mg 18.0 Ca 25
However, complacency must not set in. The application of the model made here has been undertaken on the basis of accurately sampled soil exchangeable cations and representative soil waters together with a firm hydrograph split of soil and groundwater types. However, there is con- siderable uncertainty over what the contributions from the soils are, both in terms of their average chemistries and the volumetric fluxes as shown by
STREAM WATER
5o1 1
- TWOLAYER --___ TWO BY TWO LAYER _..-.._ TWO LAYER RECALIBRATED
Fig. 4. Modelled predictions for Alkalinity (mequiv./l) and pH for the case where the stream water is modelled on the basis of the two-layer construct where the catchment is represented as a straight average of the weathering rate values times three (M3) and divided by three (D3) two-layer calibration.
M.C. Forti et al. / The Science of the Total Environment I80 (1996) 187-200 199
Table I Modelled stream, soil water and groundwater chemistry (pe- quiv./l) for the year 1984 and exchangeable cations and base saturation (%) for the years 1984 and 1844
different pattern from the two-layer application. This is the case even though the results would match baseflow, stormflow and average stream water chemistry. On this basis, and from previous findings (Hooper and Christophersen, 1992), it is important that sound measurements are obtained to characterise the contribution of the different types of water. Furthermore, while one type of het- erogeneity has been examined here, that associated with variations in weathering rate, the full hetero- geneity of the system may well be much more com- plex. For example, from the previous work of Neal et al. (1994), the variations in the adsorbed cation ratios of soil were shown to be very large indeed. While the range of values estimated in this study is large (Table 8) it is an order of magnitude or more smaller than that observed in the field by Neal et al. (1994).
43.2 25 18 0.11 0.1
Robson (1993). For example, Fig. 4 shows the modelled predictions of alkalinity and pH for the case where the stream water is modelled on the basis of the two-layer construct where the catch- ment is represented as a straight average of the D3 and M3 values. The basic information on the par- ameters set and data tit for this application are given in Tables 6 and 7. The results show a very
The actual heterogeneity of the system may well be much greater than that mimicked in this paper. Other influences such as variation in the soil selec- tivity coefficients and variations in weathering rates for individual cations may well come into play. Also, the weathering rate variations that occur within the catchment may be larger than that modelled, and this too could contribute to the differences observed.
6. Conclusions
This study indicates that variations in weather- ing rate can critically affect the variations in soil and groundwater chemistry. Despite this, the inte- gration of the variability within a simplified
Table 8 Adsorbed cations ratio values calculated for the different weathering rates
The MAX Ratio listed in the table provides a measure of the scale of variability. It is the ratio of the maximum and minimum values for the adsorbed cation ratios across the range of weathering.
200 M.C. Forti et al. / The Science of the Total Environment 180 (1996) 187-200
modelling structure may well be valid provided that the relative inputs of the various distinct chemical regions are taken into account. However, it remains critical that the relative contributions of inputs are established by field observation and in this sense the present work adds to the value of the chemical mixing approach.
Acknowledgements
One author, M.C. Forti was in receipt of a grant from ‘Conselho National de Pesquisas’ - CNPq (National Research Council - Brazil; Proc. 460073/95-8) which gave the opportunity to do this work.
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